1. Field of the Invention
This invention relates to a mobile communication system which includes a mobile station, a radio base station and an exchange system and which can cope with a radio communication system applying variable access methods, modulation methods and transmission methods. More particularly the invention relates to a mobile communication system which includes a mobile station, a radio base station and an exchange and which realizes roaming and handover between systems using different access methods.
2. Description of the Related Art
There is currently known a mobile communication system connected wirelessly between an automobile communication apparatus or a portable mobile communication apparatus (hereinafter called "mobile station") and a radio base station of a private communication system or a public communication system. The radio base station is connected to an exchange.
As multiple access methods, there are a Frequency Division Multiple Access (FDMA) method, a Time Division Multiple Access (TDMA) method, and a Code Division Multiple Access (CDMA) method.
As modulation methods, there are digital modulation methods such as a Gaussian Minimum Shift Keying (GMSK), a .pi./4-quarter differential phase shift keying (.pi./4-QDPSK), a 16-value quadrature amplitude modulation (16QAM), and a multi-subcarrier 16-value quadrature amplitude modulation (M16QAM). In addition, as the modulation method, there are analog modulation methods (e.g., an FM modulation method).
Further, in order to realize bidirectional communication, a Time Division Duplex (TDD) method, which has a time-divided transmission and reception, and a Frequency Division Duplex (FDD) method, which has a frequency-divided transmission and reception, are utilized.
FIGS. 127 through 129 are explanatory views of conventional multiple access methods shown in Electric Information Communication Handbook (page 2449, satellite communication system, the fourth section, issued Mar. 30, 1988). FIG. 127 shows a classification of the multiple access methods. FIG. 128 shows characteristics of the multiple access methods. FIG. 132 shows one example of transmission volume of the multiple access methods in case of satellite communication system.
The FDMA method is a multiple access method which divides a frequency band for transmitting and receiving signals. The TDMA method is a multiple access method which divides time for transmitting and receiving signals with a common frequency. The CDMA method is a multiple access method, in which a specific code is used to identify a communication channel. In the CDMA method, in case of transmitting signals, the specific code is allocated to each communication channel and the spread-spectrum is carried out for the same frequency carrier as the one for receiving being modulated by the code. On the receiving side, the communication channel is identified by applying a code synchronization. The CDMA method may be referred to as a spread spectrum multiple access method. The FDMA method has an advantage in that an antenna, a power amplifier, and a common carrier adapter of the land station are more simplified, compared to the TDMA method. On the other hand, the FDMA method has a disadvantage in that linearity is required in a repeater. in the CDMA method, the transmission capacity of each repeater is rather small, compared to that in the FDMA method or the TDMA method. This is due to the fact that enough gain cannot be obtained because the bandwidth of the repeater, which is installed communication satellite, is narrow and the number of channels is limited because of interference noise between carrier waves. In an automobile communication system requiring no relay, however, since there is no limit in process gain due to the narrow band of large-output relays, the CDMA method can increase the transmission capacity. Further, because of the CDMA method uses a specific code, it is excellent in keeping the communication secret and in interference-proof.
As related art, the following technical documents are identified:
1. Japanese Patent Laid-Open Publication No. Hei 6-120886, for "Mobile Communication System", filed Oct. 5, 1992. Inventors: Takahashi and Uchida. PA1 2. Japanese Patent Laid-Open Publication No. Hei 6-216836, for "Mobile Communication System", filed Jan. 20, 1993. Inventors: Uchida and Kimura. PA1 3. Japanese Patent Laid-Open Publication No. Hei 7-154859, for "Mobile Station, Exchange and Mobile Communication System", filed Nov. 29, 1993. Inventors: Uchida and Mimura. PA1 4. WO 90/13212, for "Digital Radio Communication System and Two-Way Radio". Applicant: Motorola. PA1 5. U.S. Pat. No. 5,260,967, for CDMA/TDMA Spread Spectrum Communications System and Method", filed Jan. 13, 1992 by D. L. Schilling and assigned to IDC Inc. PA1 6. U.S. Pat. No. 5,299,266, for "Adaptive Power Control for a Spread Spectrum Communications System and Method", filed Nov. 19, 1991 by D. L. Schilling and assigned to IDC Inc. discloses a mobile station which receives the spread spectrum electric wave from one land station and has a transmitter to transmit the second spread-spectrum signal corresponding to a strength of the receiving signal (AGC (Auto Gain Control) signal). PA1 7. Japanese Patent Laid-Open Publication No. Hei 5-145470, for "Multiple Access Mobile Communication System". filed Nov. 18, 1991 by NTT (Inventors: Hata Seiji, et al) discloses a system which is characterized in that the electric wave of the CDMA method of different time slots is assigned to each of the different sector cells. PA1 8. U.S. Pat. No. 5,345,467, for CDMA Cellular Hand-Off Apparatus and Method", filed Oct. 8, 1992 by G. R. Lomp, et al and assigned to IDC Inc. discloses the mobile station which received the first spread spectrum electric wave from the first land station provides another correlation receiver, such as a matching filter, to receive the second spread-spectrum signal from the second land station. PA1 9. U.S. Pat. No. 5,311,542, for "Spread Spectrum Communication System", filed Oct. 9, 1992 by Kenneth C. Eder and assigned to Honeywell Inc. discloses a system in which time multiplication is performed for the plural time division Information having preambles and then the spread-spectrum is then performed. PA1 10. U.S. Pat. No. 5,319,672, for Spread Spectrum Communication System", filed Mar. 1, 1993 by M. Sumiya, et al and assigned to KDD Co. discloses a system in which the land station transmits plural spread-spectrum electric waves and has a plurality of transmitters whose transmitting frequencies are different each other. PA1 11. WO 94/21056, for "Random Access Communication Method by Use of CDMA and System for Mobile Stations which Use the Method", filed Mar. 5, 1993 by NTT Docomo (Inventors: Umeda, et al) describes that "spread-spectrum code selection is performed by each burst" in claim 19 and subsequent claims and shows "multiple random access" in FIGS. 9 and 14. PA1 12. WO 93/19537, for "Bidirectional Communication Method in Cellular System, filed Mar. 3, 1995 by ERICSSON. PA1 13. WO 93/03558, for "Communication System with Channel Selection", filed Feb. 18, 1993 by MOTOROLA PA1 14. EP 471656, for "Cellular Mobile Radio Telephone System Reliability Enhancement", filed Feb. 19, 1992 by ERICSSON. PA1 15. EP 209185, for "Free-channel Search for Cellular Mobile Radio having Mobile Station Comparing Common-channel Reception with Interference Threshold and Identifying Interfering Fixed Station", filed Jan. 21, 1987 by PHILIP.
In each of the above three documents, only one type of multiple access method is employed in the mobile communication system. Further, only one type of modulation method is employed in the communication system.
The invention disclosed in this Motorola patent may be considered as based on the same problem as the present invention.
This U.S. Patent discloses a system in which time division multiple access (TDMA) is carried out for the synchronization code and plural data and then the spread-spectrum is performed with a chip code. According to claims 52 and 56 of the U.S. Patent, the TDMA is carried out for the spread-spectrum-processed-synchronization-code and the combined-spread-spectrum signals.
Patent Family JP62015941 filed Jan. 24, 1987.
FIG. 130 of the accompanying drawings shows a conventional correlator to be used on the receiving side of the CDMA method. In FIG. 130, a portion indicated by dotted lines is an envelope correlation network for detecting the correlation between the spread-spectrum code and the received signal. The correlator forms a noncoherent delay-lock loop using the envelope correlation network and Judges the correlation with the received spread-spectrum code using the formed loop.
However, in the conventional mobile communication system, there was a problem that the user could not select the most suitable access method among various access methods. Even if any of the foregoing related art publications proposes any similar means, it is totally silent about a more detailed configuration and method in order to realize it.
For example, the above publication 5 is silent about a time multiplication of the information of the constantly divided time slot of the TDMA method in which the spread-spectrum is not performed and the information in which the spread-spectrum is performed. It still silent that the information in which the spread-spectrum is performed by CDMA method is divided by a definite time section.
Further, in the above publication 6, two sets of receivers and transmitters are required in the mobile station when two information channels of the CDMA method are simultaneously set between two land stations and one mobile station in order to realize the seamless hand-off.
In the above publication 7, it is not realized that the electric wave of the CDMA method is set to be the same time slot of the TDMA method.
In the above publication 8, two sets of receivers, such as correlation receivers, and two sets of transmitters are required in the mobile station when two information channels of the CDMA method are simultaneously set between two land stations and one mobile station in order to realize the seamless hand-off.
In the above publication 9, compared to the above publication 5, it is new only in that the time division information has preamble.
In the above publication 10, two sets of receivers, such as correlation receivers, and two sets of transmitters are required in the mobile station when two information channels of the CDMA method are simultaneously set between two land stations and one mobile station in order to realize the seamless hand-off.
The above publication 11 is totally silent about the coexistence of the TDMA method and the CDMA method, the coexistence of different data rates, the coexistence of different chip rates, the sharing of the time frame configuration of the TDMA method and the CDMA method, and the shoring of the channel configuration of the TDMA method and the CDMA method.